Inertia and Mass:
Newton's 1st law of motion describes that "an object at rest stays at rest and an object in motion stays in motion with the similar speed in the similar direction unless acted upon by an unbalanced force." Objects tend to "keep on doing what they're doing." In fact, it is the natural tendency of objects to resists alters in their state of motion. This tendency to resist alters in their state of motion is defined as inertia.